1. Field of the Invention
This invention relates to a photoelectric sensor unit comprising a light emitting section for emitting light to a detection area, a light receiving section for receiving light from the detection area, and a signal processing section for amplifying an electric signal corresponding to the light reception amount output from the light receiving section and processing the signal.
2. Description of the Related Art
As one kind of such a photoelectric sensor unit, a sensor for detecting the presence or absence of an object to be detected in a detection area (also called a photoelectric switch) is available. A reflection-type photoelectric sensor unit detects the presence or absence of an object to be detected as light emitted from a light emitting section to a detection area is reflected on the object to be detected and is incident on a light receiving section. A transmission-type photoelectric sensor unit detects the presence or absence of an object to be detected as light emitted from a light emitting section to a detection area is blocked by the object to be detected and is not made incident on a light receiving section. Not only the photoelectric sensor units (photoelectric switches), but also a unit for detecting the shape, color, displacement, etc., of an object to be detected in a detection area by light may be called photoelectric sensor unit.
A light emitting diode (LED) is often used as a light emitting element (light source) forming a light emitting section of a photoelectric sensor unit as described above. In the related art, an LED having pn junction of a semiconductor consisting of AlGaAs (aluminum, gallium, arsenic) has been used. In the description that follows, this LED is called three-element-type LED (or simply, three-element LED).
The three-element LED has the feature of being capable of providing high brightness, but has the drawback of large life degradation. Particularly, when the LED is used in a high-humidity environment, degradation caused by oxidation of Al is large and as the LED is used for a long time, reducing of the light emission amount (brightness) is observed noticeably. Therefore, to ensure the given life of the LED, it is necessary to compensate for reducing of the brightness by a drive circuit.
That is, since the brightness of the LED becomes higher as the drive current is increased in a predetermined area, if the brightness is reduced because of life degradation, the drive current can be increased for compensating for reducing of the brightness so as to maintain constant brightness. A circuit using a monitor light receiving element for detecting the brightness of an LED and controlling the drive current of the LED so that the brightness becomes constant is called an APC (automatic output control) circuit.
A photoelectric sensor unit using an LED having pn junction of a semiconductor consisting of AlGaInP (aluminum, gallium, indium, phosphorus) as a light emitting element forming a light emitting section is also commercially practical. In the description that follows, this LED is called four-element-type LED (or simply, four-element LED).
The four-element LED is characterized by the fact that life degradation caused by oxidation of Al is small and if the LED is used for a long time, the brightness is scarcely reduced because the percent age of Al can be made small. On the contrary, the measurement result indicating that the brightness slightly rises as the LED is used for a long time is obtained. Therefore, it is found that the photoelectric sensor unit using the four-element LED does not involve a problem on practical use if an APC circuit as mentioned above is omitted.
However, the four-element LED has the drawback of low brightness as compared with the three-element LED. Thus, to provide equal brightness (light emission amount) to the three-element LED, the drive current needs to be increased. For example, a drive current of about 500 to 600 mA needs to be made to flow for the four-element LED to provide brightness as much as brightness provided when a 200-mA drive current is made to flow into the three-element LED.
To make a larger drive current flow into the LED, the current capacity of the drive circuit needs to be increased, leading to an increase in the cost of the drive circuit. Since heat generation of the drive circuit and the LED is also increased, a structure (and space) for heat radiation becomes necessary; this can become an obstacle to miniaturization of the whole unit.
In a photoelectric sensor unit using an optical fiber, to avoid the adverse effect of heat generation of an LED on the base end part of the optical fiber, a structure becomes necessary for keeping a distance between the LED and the base end part of the optical fiber and placing a condensing lens therebetween. Such a structure can also become an obstacle to miniaturization of the whole unit and a decrease in the cost.